From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Horman Subject: Re: lib: include rte_memory.h for __rte_cache_aligned Date: Tue, 9 Dec 2014 10:22:04 -0500 Message-ID: <20141209152204.GD28871@hmsreliant.think-freely.org> References: <1415381289-43291-1-git-send-email-jyu@vmware.com> <20141208150401.GB3907@localhost.localdomain> <5486B87E.5010404@6wind.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dev-VfR2kkLFssw@public.gmane.org To: Olivier MATZ Return-path: Content-Disposition: inline In-Reply-To: <5486B87E.5010404-pdR9zngts4EAvxtiuMwx3w@public.gmane.org> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces-VfR2kkLFssw@public.gmane.org Sender: "dev" On Tue, Dec 09, 2014 at 09:53:18AM +0100, Olivier MATZ wrote: > Hi Neil, > > On 12/08/2014 04:04 PM, Neil Horman wrote: > >On Fri, Nov 07, 2014 at 09:28:09AM -0800, Jia Yu wrote: > >>Include rte_memory.h for lib files that use __rte_cache_aligned > >>attribute. > >> > >>Signed-off-by: Jia Yu > >> > >Why? I presume there was a build break or something. Please repost with a > >changelog that details what this patch is for. > >Neil > > I don't know if Yu's issue was the same, but I had a very "fun" issue > with __rte_cache_aligned in my application. Consider the following code: > > struct per_core_foo { > ... > } __rte_cache_aligned; > > struct global_foo { > struct per_core_foo foo[RTE_MAX_CORE]; > }; > > If __rte_cache_aligned is not defined (rte_memory.h is not included), > the code compiles but the structure is not aligned... it defines the > structure and creates a global variable called __rte_cache_aligned. > And this can lead to really bad things if this code is in a .h that > is included by files that may or may not include rte_memory.h > > I have no idea about how we could prevent this issue, except using > __attribute__((aligned(CACHE_LINE))) instead of __rte_cache_aligned. > > Anyway this could probably explain the willing to include rte_memory.h > everywhere. > > Regards, > Olivier > > So, that is a great explination, and would be good to have in the changelog. Also, to avoid the problem that you describe, while its preferred to have it at the end of a struct, you can also put the alignment attribute right after the struct keyword in gcc: https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax That seems like it would solve the problem going forward. Neil